1. Field of the Invention
The present invention relates to a digital TV receiver, and more particularly, to an apparatus for recovering a carrier in a VSB system digital TV receiver.
2. Background of the Related Art
The VSB system, employed as Korean and the USA digital TV (DTV) broadcasting standards, is designed to transmit a broadcasting signal by using a frequency allocated for the present analog TV broadcasting. However, for minimizing an influence to the present analog TV broadcasting, the DTV signal is transmitted at a very low intensity compared to the analog TV signal. Of course, the standards are set such that there is no problem in reception of the DTV signal even if the intensity of the signal is low by employing different coding system in the DTV signal and channel equalizers for reducing an influence from noise. However, if a situation of the transmission channel is very poor, the reception of the signal is very poor. In general, since the DTV transmission system has an advantage in that a noise occurred on a transmission channel is removed perfectly at reception of the broadcasting signal, to enable to watch a picture having no noise at all, while the DTV transmission system has a disadvantage in that the picture can not be watched at all if the transmission signal is not recovered fully, it is required that the DTV receiver receives whole signal whatever poor transmission channel the signal has passed.
FIG. 1 illustrates a block diagram of a related art VSB system DTV receiver, wherein, upon reception of a RF (Radio Frequency) signal modulated in a VSB system through an antenna 101, after selecting a particular channel frequency the user desires, a tuner 102 converts the RF band VSB signal carried on the channel frequency into an IF band (Intermediate Frequency band) signal (in general 44 MHz or, in the case of analog TV broadcasting, 43.75 MHz is used widely), and filters out other channel signals, appropriately.
The signal from the tuner 102, which converts a spectrum of channels into an IF pass band signal, passes a SAW (Surface Acoustic Wave) filter 103 employed for removing neighboring channel signals, and the noise signal.
In this instance, since the digital broadcasting signal has all information within a 6 MHz zone centered on, for an example, the IF of 44 MHz, the SAW filter 103 removes other zones from the output of the tuner 102, only leaving the 6 MHz zone having the information, and provides to an IF amplifier 104.
The IF amplifier 104 multiplies a gain calculated beforehand to the output from the SAW filter 103 for making an amplitude of the signal from an A/D converter 105 after the IF amplifier 104 the same always. Accordingly, the A/D converter 105 receives and digitizes signals of the same amplitude from the IF amplifier 104. The passband signal digitized at the A/D converter 105 is transited to a baseband signal at the carrier recoverer 106, and provided to a DC remover 107. In this instance, the carrier used in recovering the carrier at the carrier recoverer 106 is turned to a DC component having 0 Hz frequency.
That is, the DC component is forcibly inserted into a transmission signal at a transmitter so as to perform the carrier recovery at the carrier recoverer. Therefore, after the carrier recovery is done, the DC component inserted at the transmitter is required no more. According to this, the DC remover 107 detects and removes the DC component from the baseband signal from the carrier recoverer 106. The baseband digital signal having the DC component removed therefrom is provided both to a synchronizer 108 and a channel equalizer.
The most remarkable features of the VSB transmission system suggested by the Grand Alliance (GA) compared to other DTV transmission system are a pilot signal, a data segment synchronizing signal, and a field synchronizing signal. The signals are inserted at the transmitter before transmission for improving characteristics of carrier recovery and timing recovery.
Accordingly, the synchronizer 108 recovers the data segment synchronizing signal, and the field synchronizing signal from the signal inserted at the time of transmission having the DC component removed therefrom. The synchronizing signals obtained thus are provided to the channel equalizer 109, a phase corrector 110, and an FEC 111.
The channel equalizer 109 removes a linear distortion of an amplitude in the baseband digital signal causing interference between symbols, ghost occurred as the signal is reflected at buildings or mountain, and the like by using the baseband digital signal and the synchronizing signal, and provides to the phase corrector 110.
Referring to FIG. 1, the signal passed through all analog processes is converted into a digital signal at the A/D converter 105 and provided to the carrier recoverer 106. Therefore, all of the digital process blocks after the carrier recoverer 106 cannot make regular operation if the carrier recovery is not made at the carrier recoverer 106.
FIG. 2 shows characteristics of an airwave signal frequency defined as present DTV standards in Korea and in the United States. Center frequency is transcribed as fc, and pilot frequency is transcribed as fp. A center frequency and a pilot frequency are different in each channel. For example, a frequency in the center of 6 MHz width of each ground wave channel is the center frequency, and a frequency having carrier signal on the transmission signals is called the pilot signal. Pilot is a carrier having a reduced size for DTV signals not to interfere analog TV signals.
If there is no linear ghost in the signals inputted to the DTV receiver, the carrier recoverer 106 is able to recover a location of the pilot frequency on the signal frequency because the amplitude of data and the pilot is consistent.
However, if there is linear ghost in the signals inputted to the DTV receiver, the relative amplitude of data and pilot is changed by delaying time of the linear ghost and phase difference.
FIGS. 3a and 3b are diagrams showing shape of pass band frequency when time delay of ghost is generated in a 1st symbol section. FIG. 3a shows a frequency characteristic when the phase difference of ghost is 0°, and FIG. 3b shows a frequency characteristic when the phase difference of the ghost is 180°. Compared to the frequency characteristic in FIG. 2, the amplitude of pilot is larger than that of data in the case of FIG. 3. On the contrary, the amplitude of pilot is smaller than that of data in FIG. 3b. 
FIGS. 4a and 4b shows a shape of pass band frequency in case that time delay of ghost is occurred in about 10 symbol section. As illustrated in FIG. 4a, if the phase difference is 0°, the amplitude of pilot is relatively larger than that of data. And, When the phase difference is 180° as illustrated in FIG. 4b, the amplitude of pilot is smaller than that of data.
As described above, a size of the pilot signal changes in case that there is linear ghost in signals inputted to the DTV receiver. Therefore, accurate carrier recovery can not be carried out.